Apparatus for making non-woven fibrous sheet

ABSTRACT

An improved apparatus for use in making nonwoven fibrous sheets of organic synthetic polymers includes a spinneret orifice through which a polymer solution is flash spun to form a plexifilamentary strand directed in a generally horizontal direction toward a rotating baffle whose axis of rotation is generally parallel to but spaced from the axis of the extrusion orifice and whose surface is contoured so as to simultaneously spread said strand into a planar web, direct the web into a generally vertical plane downward toward a collecting surface, and cause said web to oscillate in the plane as the baffle rotates. The improvement consists of an aerodynamic shield of specified configuration. The shield terminates at an edge which lies substantially along an arc of a circle whose center lies on the axis of rotation of the baffle. Since the plexifilamentary strand impinges on the baffle at a point near its axis of rotation and is deflected as a web which therefore oscillates through various radial directions within the plane substantially perpendicular to the rotation axis, only the specified shield configuration will provide flow paths of approximately equal impedance, independent of the instantaneous radial direction at which the web leaves the baffle.

United States Patent [1 1 Brethauer et al.

[ Jan. 14, 1975 APPARATUS FOR MAKING NON-WOVEN FIBROUS SHEET [75]Inventors: Dale Merrill Brethauer, Wilmington,

Del.; Jean Paul Prideaux, Richmond, Va.

[73] Assignee: E. l. Du Pont de Nemours and Company, Wilmington, Del.

[22] Filed: Mar. 1, 1974 [21] Appl. No.: 447,253

Related U.S. Application Data [62] Division of Ser. No. 303,044, Nov. 2,1972.

[52] U.S. Cl 425/3, 264/22, 264/24, 425/224, 425/231, 425/456, 425/373[51] Int. Cl B29j 5/00 [58] Field of Search 264/5, 22, 176, 24; 425/83,72, 75, 162, 224, 231, 456, 3, 373

[56] References Cited UNITED STATES PATENTS 3,387,326 6/1968 Hollberg etal. 264/24 X 3,456,156 7/1969 Kilby et al. 264/22 X 3,490,1l5 l/l970Owens et al. 264/24 X 3,497,918 3/1970 Pollock, Jr. et al 425/3243,549,453 12/1970 Smith 425/19l 3,565,979 2/1971 Palmer 264/24 PrimaryExaminer-Robert L. Spicer, Jr.

[57] ABSTRACT An improved apparatus for use in making nonwoven fibroussheets of organic synthetic polymers includes a spinneret orificethrough which a polymer solution is flash spun to form aplexifilamentary strand directed in a generally horizontal directiontoward a rotating baffle whose axis of rotation is generally parallel tobut spaced from the axis of the extrusion orifice and whose surface iscontoured so as to simultaneously spread said strand into a planar web,direct the web into a generally vertical plane downward toward acollecting surface, and cause said web to oscillate in the plane as thebaffle rotates. The improvement consists of an aerodynamic shield ofspecified configuration.

The shield terminates at an edge which lies substantially along an arcof a circle whose center lies on the axis of rotation of the baffle.Since the plexifilamentary strand impinges on the baffle at a point nearits axis of rotation and is deflected as a web which thereforeoscillates through various radial directions within the planesubstantially perpendicular to the rotation axis, only the specifiedshield configuration will provide flow paths of approximately equalimpedance, independent of the instantaneous radial direction at whichthe web leaves the baffle.

7 Claims, 4 Drawing Figures TO SOLVENT -44 R EC OVERY SUPPLY POSITIVE OR,135 NEGATIVE DC SOURCE 'GROUNDED DC SOURCE PAIENIED JAN 1 41975 SHEET 1or 3 I 20 2 TO SOLVENT Fl 6- 1 fl RECOVERY II IO 8 2\3 1' w POLYMERSOLUTION SUPPLY POSITIVE OR /35 NEGATIVE 0c '6 SOURCE 'GROUNDED OR DCSOURCE v Q) 36 i U D D 3? ./29O F I 6- 3 29b PATENTED JAN I 4 I975 SHEEI2 OF '3 APPARATUS FOR MAKING NON-WOVEN FIBROUS SHEET CROSS REFERENCE TORELATED APPLICATION This is a division of application Ser. No. 303,044filed Nov. 2, 1972.

BACKGROUND OF THE INVENTION This invention relates to a process andapparatus used in the preparation of nonwoven fibrous sheets ofsynthetic organic polymers. More particularly, it is directed to aprocess and apparatus for spreading a plexifilamentary strand into aplanar web, directing the web toward a surface, charging the web andcollecting the web in the form of a nonwoven fibrous swath.

Steuber, U.S. Pat. No. 3,169,899 describes a process for making anonwoven sheet from flash-spun fibrous materials. In the flash spinningtechnique, a solution of an organic polymer which is under pressure andat a temperature far above the boiling point of the solvent is extrudedinto an area of substantially atmospheric pressure. As the materialissues from the orifice, the solvent flash evaporates and aplexifilamentary strand is formed. The plexifilamentary strand iscomposed of very thin film fibril elements which are interconnected in athree-dimensional network as described in detail in Blades and White,U.S. Pat. No. 3,081,519. The threedimensional network is spread into awide web by caus ing it to be swept along a smooth path past a curvedsurface baffle whereupon the expanding solvent gas spreads the material.By oscillating the deflecting baffle, the web can be directed to variousareas across the width of a moving collecting belt where it is depositedin the form of swaths. The web can be electrostatically charged to bothfurther increase its width through mutual electrostatic repulsionbetween fibrils and also attract the swath to the belt and immobilizethe deposit. A fibrous nonwoven sheet is thereby obtained.

In an alternative process described in Pollock and Smith, U.S. Pat. No.3,497,918, the oscillating baffle can be replaced by a rotating baffle,having specially contoured surfaces, which simultaneously spreads andoscillates the web as it is directed through an electrostatic device toapply uniform electrostatic charge on the web and promote uniformdeposition of the web on a moving collecting surface. A suitablecharging apparatus is described in Kilby and Smith, U.S. Pat. No.3,456,156. The apparatus consists of an annular disc target electrodewhich is concentric with the rotating baffle and rotates independentlyof said baffle. A multineedle ion gun is positioned opposite the targetplate, the needles being aimed at a portion of the target electrode toprovide a corona discharge zone. The fibrous material moving in a planarpath between the target electrode and the ion gun needles iselectrostatically charged before being deposited on the movingcollecting surface.

A number of requirements must be satisfied in order to obtain wide,fibrous, nonwoven sheets having a uniform appearance and a uniform basisweight. In general, wide nonwovens are obtained by blending andoverlapping the swaths from several spinning positions. Smith, U.S. Pat.No. 3,549,453, describes a mechanism for making fine adjustments andvarying the weight distribution of the swaths deposited on thecollection surface. Tests have shown that optimum basis weightuniformity in the cross machine direction (i.e,, the direction at rightangles to the direction of movement of the receiving surface) isobtained when the width of the swaths at this surface is within certainlimits which depend on the shape of the cross machine direction basisweight profile of each swath. This width is a function of the amplitudeof the oscillation imparted to the web by the baffle, the amountofelectrostatic charge on the web and the distance between the baffleand the receiving surface.

lsakoff, U.S. Pat. No 3,593,074, describes a short diffuser device,termed a "scoop", which squeezes the gaseous stream to increase itswidth, thereby also increasing the width of the entrained web. Thisscoop is situated between the baffle and the corona charging station,and leads to improvements in sheet uniformity by permitting shorterbaffle-to-receiving surface distances for a given swath width. However,as spinning throughputs are increased, the larger volumes and velocitiesof gas produced in the flash spinning operation create an undesirableincrease in turbulence. This increases the random oscillation of the webproducing a nonwoven sheet having less than the desired uniformity.

SUMMARY OF THE INVENTION An apparatus is provided for forming a fibrousweb that includes means for flash spinning a polymer solution to form aplexifilamentary strand entrained in a gaseous stream, means at onelocation (e.g., a rotating baffle) for spreading the strand to form aweb and oscillating the web in a generally vertical plane in a pluralityof downward radial directions toward a collecting surface and meanspositioned below said spreading and oscillating means for charging saidweb. The improvement comprises an aerodynamic shield having front andrear members disposed on each side of said plane below said spreadingand oscillating means. The members have surfaces facing said plane, saidsurfaces terminate in edges equispaced from each other that lie alongarcs of equal radius extending from a horizontal axis proximate to saidone location. The surface of the rear member facing said plane issubstantially parallel thereto and is a stepped surface, stepped awayfrom said plane in the downward direction along one or more arcsconcentric with the terminating edge of the rear member. The surface ofthe front member facing said plane is a section of a surface ofrevolution about said horizontal axis that converges downwardly towardsaid plane.

The surface of the front member facing the vertical plane preferably isa segment of a right circular cone converging downwardly toward saidplane at an angle of about 5.

The front and rear members extend downward for a distance of from about30 percent to about percent of the distance from said one location tosaid collecting surface.

There are ports in said rear member at the location the surface of therear member facing the vertical plane is stepped away from said plane.

The invention concerns an improved process that includes the steps ofentraining a web in a gaseous stream flowing in a generally horizontalpath toward one location, directing and oscillating said web and saidstream from said one location in a plurality of downward radialdirections in a substantially vertical plane through ambient gas towarda collecting means, and collecting said web on said collecting means asa fibrous sheet, the improvement comprising: converging said stream insaid downward radial directions within a shield presenting substantiallyequal flow impedances in said radial directions. The process may includethe step of aspirating said ambient gas into said shield generally insaid downward radial directions.

Although some web deceleration can be tolerated within the aerodynamicshield of this invention, the primary function of the shield is not todiffuse the gas stream but rather to protect it and the entrained web asshaped by the rotating baffle and prevent premature mixing with theambient gas. Accordingly, the exit gap width between front and rearshield members is selected such that there is neutral gas pressure onthe members, i.e., the average internal and external gas pressures areequal. The actual gap width required is, of course, a function of therate of gas generation by flash vaporization, the quantity of gasentrained and aspirated into the shield, the dimensions of the shield,etc. However, under any given set of spinning conditions the requiredgap width is readily determined by the neutral pressure condition.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional elevationindicating schematically the arrangement of various elements of anapparatus which can be used in the practice of the invention.

FIG. 2 is a more detailed cross-sectional view of a portion of apreferred embodiment of the aerodynamic shield of the present invention.

FIG. 3 is a view of the web facing surface of the front shield member ofFIG. 2.

FIG. 4 is a graph presentation of data showing the improved machinedirection sheet uniformity provided by the apparatus and process of thepresent invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS Referring to FIG. 1,a spinneret device 1 is shown connected to a polymer solution supplysource. Polymer solution 2 under pressure is fed through an orifice 3into intermediate pressure or letdown pressure zone 4 and then throughspinning orifice 5 into web forming chamber 6. The extrudate fromspinning orifice 5 is a plexifilamentary strand 7. Due to the pressuredrop at spinning orifice 5 and the high temperature of the spinningsolution, vaporization of solvent creates a vapor blast which, bypassage along the surface of baffle 8 concomitantly with plexifilament7, generally follows the path of advance from spinning orifice 5 tocollecting surface 9, thereby creating a flow pattern within chamber 6as indicated by the arrows in FIG. 1. Baffle 8 is mounted on shaft 10which is mounted in bearing 11 and is rotated by means not shown. Thesurface of baffle 8 is so contoured that the plexifilamentary strand 7issuing from orifice 5 is deflected into a generally vertical plane andsimultaneously spread laterally to form a plexifilamentary web 21 whichoscillates from sideto-side as baffle 8 is rotated.

The plexifilamentary web 21 passes from baffle 8 directly into theaerodynamic shield of this invention. The shield is comprised of frontmember 18 and a rear member comprising elements 13 and 17. Multineedleion gun 14 is mounted on the interior surface of front member 18, and isconnected to constant current power source 35 which supplies a potentialof approximately 50-60 kilovolts. A corona discharge occurs betweenneedles 14 and target plate 13 which is disposed so that the vapor blastoriginating at 5 and deflected by baffle 8 series the plexifilament webalong its charging surface. Target plate 13 is connected via commutatingring and brushes to ground by wire 15 and microammeter 16 whichindicates target plate current.

Target plate 13 is an annular metal disc electrode, and is preferablycovered with a dielectric insulating surface as disclosed in US. Pat.No. 3,578,739. Target plate 13 together with concentric annular segment17 comprise the rear member of the aerodynamic shield, and are adaptedto be rotated concentrically with, but independent of, baffle 8 by meansnot shown. During rotation of the rear member, its interior surfacepasses by rotating brush 20, driven by means not shown, so that thesurface of target plate 13 and adjacent parts may be cleared of anydebris, thereby furnishing a continuously cleaned surface for optimumoperation of the corona discharge. At intervals, in a circular pattern,the rear shield member is pierced by ports 19 through which ambient gasmay be aspirated into the step region between concentric disc segments13 and 17.

After exiting the aerodynamic shield, plexifilament web 21 is depositedupon a collecting surface 9. The surface illustrated is a continuouselectrically conductive belt forwarded by drive roll 36. The belt mayeither be grounded or charged to a positive or negative potential bypower source 37. Due to differences in their electrostatic charge, theplexifilament web 21 is attracted to surface 9 and clings to it in itsarranged condition as a swath 38 with sufficient force to overcome thedisruptive influences of whatever vapor blast may reach this area. Sincehigh rates of production can generate high turbulence at chamber 6,auxiliary corona devices 43 stationed just above the surface of belt 9may be employed to place even higher electrostatic charge on swath 38,thereby pinning it even more tightly to belt 9. Wide sheets are producedby blending and overlapping the output from several spinning positionsplaced in an appropriate manner across the width of a receiving surfacesuch as the belt 9. The degree of uniformity of the web deposits tendsto increase as the aerodynamic shield is extended to protect a greaterfraction of the vertical distance from the baffle to the receivingsurface. However, unless some free-fall is permitted, the web fibrilorientation is biased too heavily in the cross machine direction.Furthermore, too extensive a shield can lead to nonuniform deposits and37 hang-up due to electrostatic attraction and erratic clinging of thecharged web to the internal shield surfaces. Applicants have discoveredthat a preferred balance in sheet uniformity and fibril directionalityis attained when the aerodynamic shield extends over about 30-60 percentof the vertical distance from the edge of the baffle to the collectingsurface. The sheet is then lightly compacted by roll 41 and is collectedon windup roll 42 after passing through port 39 and flexible elements(or rolls) 40 which assist in retention of vapor within chamber 6. Aconventional solvent recovery unit 44 may be beneficially employed toimprove economic operation.

FIG. 2 is an enlarged cross-sectional view of a portion of theaerodynamic shield depicted in FIG. 1. Front member 18 is constructed ofLucite (trademark for Du Pont acrylic resin), its exit lip 22 lies alonga semicircle centered on rotating baffle axis 23 and the angle ofconvergence 24 is 5. Multineedle corona charging electrode 14 consistsof 17 needles smoothly graduated in length, spaced at intervals alongconductor 25 to provide a varying needle-to-needle target spacing. Theneedle electrode is recessed in channel 31 cut into the web facingsurface of front member 18. The needles lie along an arc of a circlecentered on axis 23, and the angle subtended by the end needles asviewed from the center of the circle is l40, which is sufficient tooverlap the plexifilamentary web envelope throughout its period ofoscillation. (The angle subtended by channel 31, viewed from the centerof the circle is 166.) The 17 needles pass through a curved strip ofLexan 26 whose width is tapered to allow each needle point to protrudethe same distance. This strip thus bridges the gap between adjacentneedles and prevents plexifilamentary debris from building up from thebase of the individual needles during spinning, which debris could alterthe shape and intensity of the electrical field and thereby lead tononuniform corona generation and consequent erratic web charging andsheet nonuni formities.

The rear shield member is comprised of metallic annular target plate 13,coeered by a dielectric surface 27 having a volume resistivity between 5X and 1 X 10 ohm centimeters, and annular target plate extension 28constructed of Lucite plus concentric annular segment 17, alsoconstructed of Lucite. These rear shield members elements are assembledby means of annular Lexan (General Electrics polycarbonate resin)support member 30 and thereby adapted to be rotated as a unit about axis23. Le, concentrically with baffle 8. (Baffle 8 is rotated at muchhigher speeds than the rear shield unit). The step height betweenelements 28 and 17 is designated 19a, and support member 30 isperforated at intervals by aspiration ports 19. The provision of accessports in the rear member near the step permits a suitable quantity ofambient gas to be aspirated into the aerodynamic shield to form aprotective cushion of gas which flows toward the exit lip along theinternal surface of the rear member.

FIG. 3 is a view of the web facing surface of front shield member 18showing the needle electrode unit 14, channel 31, Lexan strip 26, andalso indicating support members 29a and 29b. Front shield member 18 issuspended from pivot points (not shown) inside members 29a and 29b, bothlocated on a common horizontal axis, which thus permits front shieldmember 18 to swing farther away from the rear shield member momentarilywhenever a mass of plexifilamentary material may begin to plug theshield passageway. Also mounted within members 29a and 29b are metalspring elements (not shown) urging front member 18 back into normaloperating position as soon as the plug mass is expelled. Adjustablestops (not shown) are provided such that the normal exit gap width 22ais set in the range of from Va to inch, or the value at which no net gaspressure difference exists on front element 18 in a direction parallelto axis 23. The specific neutral pressure gap with width may beestablished during spinning, for example, by temporarily releasing thefront member from the spring biasing means and allowing it to swing freeon its pivots and seek a position such that pressure on the web side (orinside") is in balance with the ambient gas pressure on the oppositeside (outside"). Of course, appropriate allowance or correction may needto be made should the center of gravity of the front shield member notlie directly beneath the pivot points. As may be noted from FIGS. 2 and3, the entrance edge of front member 18 has a configurationapproximating a section of a toroid, thus surrounding a portion ofbaffle 8 and extending backward to a point close to the spinnerethousing in order to restrict entrainment of ambient gas while stillproviding sufficient clearance of the occasional swinging motionrequired to relieve adventitious plugs.

The web facing surface of the front member of the aerodynamic shieldmust satisfy two requirements: I) it should present approximately equalgas flow impedance at all radial directions at which the web may leavethe rotary baffle in the vertical plane, and (2) the spacing betweenconfronting surfaces of front and rear members should decrease on theaverage as the web moves radially through the shield from entrance toexit. This decreasing separation between shield members partiallycompensates for the increase in width of the gas stream as it flowsalong diverging radial directions within the vertical plane as directedby the contoured surface of the rotating baffle, and thereby keeps thecross-sectional area of the gas stream taken at right angles to itsdirection of flow from increasing so rapidly that excessive decelerationand sticking of the web to the shield can occur. The equal impedancecriterion is met by a front shield member whose web facing surface is asection of a surface of revolution about the rotation axis of thebaffle; and since the rear members surface is approximately flat (apartfrom the annular steps), the convergence criterion can simultaneously besatisfied most simply by choosing the surface of the front member to bea segment of a right circular cone. The optimum angle of convergence,(i.e., the angle between the vertical and a straight line tangent to theweb-facing surface of the front member at two points proximate itsentrance and exit edges, respectively, measured on a vertical crosssection perpendicular to the web oscillation plane) is a function of thedetailed contour of the baffle surface, the number and size of the stepsin the rear shield member, etc. Fortunately, this convergence angleappears to be not too sensitive to variations in these parameters and aconvergence angle of 5 has been found to be generally satisfactory.Various materials, e.g., Lucite, nylon, Teflon (trademark for Du Pontfluorocarbon resin), various filled nylons and Teflons, Nema G(laminated glass/epoxy insulating materials), Lexan, etc., can be usedfor members 17, 18, 26, 28 and 30, if desired.

In order to illustrate the improved uniformity of nonwoven sheets madepossible by the present invention, particularly at higherproductivities, a series of samples is prepared employing prior artapparatus and compared with a series of samples prepared with thepresent apparatus, using perecent coefficient of variation of basisweight uniformity as the criterion.

PERCENT CV OF BASIS WEIGHT UNIFORMITY A sheet of material about 500inches long and at least 8 inches wide is used. Eighty l-inch diametercircles are cut from the sheet along each of three rows, thecenter-center distances of these circles being about 3 inches in thewidth direction of the sheet and 6 inches in the length direction. Thecoefficient of variation (percent CV) of the weights of these l-inchcircles in each row is calculated and the average percent CV for thethree rows is used as a measure of the sheet uniformity.

EXAMPLE Linear polyethylene having a density of 0.95 g./cc. and a meltflow rate of 0.9 gram/ minutes as determined by ASTM method D-l238-57T,condition E, is flash spun from a hot trichlorofluoromethane solution.The solution is continuously pumped to the spinneret assembly under highpressure. The solution then passes into a small chamber through a firstorifice to reduce the pressure to the desired value for flash spinning,and is then immediately extruded into a region at substantiallyatmospheric pressure through a second (spin) orifice. Initial flashvaporization occurs inside a short cylindrical tunnel" immediatelydownstream and coaxin] with the spin orifice, which serves to shape theresulting high velocity gas and entrained strand. The resultingplexifilamentary strand passes along the surface ofa rotating bafflewhich simultaneously spreads it, imparts lateral oscillation and directsit vertically downwards through a corona charging zone between amultineedle corona discharge electrode and a grounded target platetoward a moving belt, where it is collected in overlapping layers. Thesheet is then lightly consolidated by passage between a pair of rollsunder a pressure of about 45 lb./lineal inch. The speed of the laydownreceiver is adjusted incrementally to obtain a set of nonwoven sheetsamples of graduated basis weights.

Two sets of plexifilamentary sheet samples are prequired to obtain a 252 /2 inch swath width for the plexifilamentary deposits on the belt. Inaddition, maximum possible sheet uniformity is sought by applying thehighest electrostatic charge level to the web which can be toleratedwithout causing web hangups", disruptive lightening discharges, etc. Thespecific parameters employed for each sample are listed in Table l. Allsamples in set A are prepared employing a spin orifice sized to yield anominal polymer throughput of 100 lbs./hr., while all samples in set Bare prepared with a smaller orifice providing a nominal lbs/hr. ofpolymer.

The percent coefficient of variation of basis weight uniformity isdetermined for the sheet samples within each set, as listed in Table I,and the data are presented graphically in FIG. 4. Low values of percentCV, corresponding to the greatest degree of uniformity are, of course,preferred. It is apparent on comparing the curves for sample sets A andB, that substantially superior sheet uniformity is provided by theapparatus and process of the present invention, than can be achieved bythe best prior art technology. This has been accomplished even at higherrates of productivity versus 75 lbs/hr.) which, except for the benefitsprovided by the present invention, otherwise inherently lead to deggadation in sheet uniformity. It has also been observed that theapparatus of the present invention provides the further advantages ofless variable swath width from run-to-run and day-to-day, andsubstantially decreases sensitivity to other process variables such asequipment alignment, web charge, etc.

TABLE 1 Set A: Aerodynamic Shield, Baffle Edge to Belt 13% inches, SpinOrifice 52 i A mils.

Solution Tunnel Flow Rate Swath Width Web Charge Basis Wt. %CV Samp.Cone. Temp. Pres. L/D (pph polymer) (inches) (microcoulombs/ (ozJyd C.psig gm.)

1 12.6% 184 980 0.300"/0.300" 100.7 23.5 10 2.02 6.45 2 do. do. do. do.do. do. 11 2.04 6.29 3 do. do. do. do. do. do. 12 1.97 6.91 4 do. do.do. do. do. 24 13 1.92 6.95 5 do. 1000 do. 105.0 23.5 11 1.98 7.15 6 do.do. do. do. 104.6 23.5 11 0.98 9.52 7 do. 186 do. do. 102.6 26 11 0.919.77 8 do. do. do. do. do. 26.5 11 2.07 6.71 9 do. do. do. do. do. 120.91 9.56 10 do. do. do. do. do. 27 13 0.86 9.70

Set B:

scoop, Baffle Edge to Belt 11% inches, Spin Orifice 44 i 2 mils.

11 12.6% 185 1060 0.266"/0.266" 70.6 27.5 11.8 2.01 7.50 12 do. 186 do.0.234"/0.234" 68.6 26 11.5 2.15 8.17 13 12.2% do. 1000 do. 72.6 22.511.7 0.91 12.44 14 do. do. do. do. do. 23.5 11.1 0.95 12.01 15 12.6% 1871100 0.234"/0.266" 85.3 24.5 10.1 1.15 10.94 16 do. 186 1060 do. 76.6 2611.9 2.14 8.24 17 do. do. do. do. do. 26 11.9 2.14 7.60

pared. Set A is prepared employing the aerodynamic shield of the presentinvention, using the preferred embodiment detailed in FIGS. 2 and 3, andhaving exit and entrance radii of 9 /2 and 2% inches, respectively, witha step height of 5/32 inch. For comparison, set B is prepared employingthe best available prior art apparatus, namely, the scoop diffuserdisclosed in US. Pat. No. 3,593,074 and in particular the embodimentdescribed in Example 4 of the patent. For each set of samples theoptimum available baffle designs are employed, together with theappropriate baffle-to-belt distance re- What is claimed is:

1. In an apparatus for forming a fibrous web that includes means forflash spinning a polymer solution to form a plexifilamentary strandentrained in a gaseous stream, means at one location for spreading thestrand to form a web and oscillating the web in a generally verticalplane in a plurality of downward radial directions toward a collectingsurface and means positioned below said spreading and oscillating meansfor charging said web, the improvement comprising: an aerodynamic shieldhaving front and rear members disposed on each side of said plane belowsaid spreading and oscillating means, said members having surfacesfacing said plane, said surfaces terminating in edges equispaced fromeach other and lying along arcs of equal radius extending from ahorizontal axis proximate to said one location, the surface of the rearmember facing said plane being substantially parallel to and being astepped surface stepped away from said plane in the downward directionalong one or more arcs concentric with the terminating edge of the rearmember, the surface of the front member facing said plane being seasection of a surface of revolution about said horizontal axis convergingdownwardly toward said plane.

2. The apparatus as defined in claim 1, the surface of the front memberfacing the vertical plane being a segment of a right circular coneconverging downwardly toward said plane at an angle of about 3. Theapparatus as defined in claim 1, said front and rear members extendingdownward for a distance of from about 30 percent to about 60 percent ofthe distance from said one location to said collecting surface.

4. The apparatus as defined in claim 3, said one location being arotating baffle, said front and rear members extending downward for adistance of from about 30 percent to about 60 percent of the distancefrom the edge of the baffle to said collecting surface.

5. The apparatus as defined in claim 1, there being through ports insaid rear member at the location the surface of the rear member facingthe vertical plane is stepped away from said plane.

6. The apparatus of claim 5, said ports being equispaced from each otheralong an arc concentric the terminating edge of the rear member.

7. The apparatus as defined in claim 1, said means for charging the webbeing incorporated in said shield.

1. In an apparatus for forming a fibrous web that includes means forflash spinning a polymer solution to form a plexifilamentary strandentrained in a gaseous stream, means at one location for spreading thestrand to form a web and oscillating the web in a generally verticalplane in a plurality of downward radial directions toward a collectingsurface and means positioned below said spreading and oscillating meansfor charging said web, the improvement comprising: an aerodynamic shieldhaving front and rear members disposed on each side of said plane belowsaid spreading and oscillating means, said members having surfacesfacing said plane, said surfaces terminating in edges equispaced fromeach other and lying along arcs of equal radius extending from ahorizontal axis proximate to said one location, the surface of the rearmember facing said plane being substantially parallel to and being astepped surface stepped away from said plane in the downward directionalong one or more arcs concentric with the terminating edge of the rearmember, the surface of the front member facing said plane being seasection of a surface of revolution about said horizontal axis convergingdownwardly toward said plane.
 2. The apparatus as defined in claim 1,the surface of the front member facing the vertical plane being asegment of a right circular cone converging downwardly toward said planeat an angle of about 5*.
 3. The apparatus as defined in claim 1, saidfront and rear members extending downward for a distance of from about30 percent to about 60 percent of the distance from said one location tosaid collecting surface.
 4. The apparatus as defined in claim 3, saidone location being a rotating baffle, said front and rear membersextending downward for a distance of from about 30 percent to about 60percent of the distance from the edge of the baffle to said collectingsurface.
 5. The apparatus as defined in claim 1, there being throughports in said rear member at the location the surface of the rear memberfacing the vertical plane is stepped away from said plane.
 6. Theapparatus of claim 5, said ports being equispaced from each other alongan arc concentric the terminating edge of the rear member.
 7. Theapparatus as defined in claim 1, said means for charging the web beingincorporated in said shield.